System and method for estimating battery state of charge

ABSTRACT

A battery system is disclosed herein. The battery system includes a battery and a controller operatively connected to the battery. The controller is configured to estimate a battery state of charge based on a history of the battery current such that the battery state of charge estimate is obtainable during the battery&#39;s course of use. A corresponding method for estimating the state of charge of a battery is also disclosed.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a system and methodadapted to estimate a battery's state of charge.

Battery state of charge (SOC) generally refers to the battery'sremaining capacity. Knowing the amount of energy left in a battery givesthe user an indication of how much longer a battery will continue toperform before it needs to be recharged or replaced. This informationmay be particularly important for applications in which excessivebattery depletion must be avoided to ensure the device remains fullyoperational at all times.

There are several methods and systems for estimating SOC. One problem isthat variables such as the rate at which the battery has been charged ordischarged over time can introduce imprecision into conventional methodsfor estimating SOC. Another problem is that some conventional methodsfor estimating SOC require a steady state condition wherein the batteryhas not been charged or discharged for a period of several hours. It canbe seen that these methods are not optimal for implementation withsystems in which the battery is frequently being either charged ordischarged, or being charged or discharged with a varying current.

BRIEF DESCRIPTION OF THE INVENTION

The above-mentioned shortcomings, disadvantages and problems areaddressed herein which will be understood by reading and understandingthe following specification.

In an embodiment, a battery system includes a battery and a controlleroperatively connected to the battery. The controller is configured toestimate a battery state of charge based on a history of the batterycurrent such that the battery state of charge estimate is obtainableduring the battery's course of use.

In another embodiment, a patient monitoring system includes a battery, apatient monitoring device operatively connected to the battery, and acontroller operatively connected to the battery. The controller isconfigured to estimate a battery state of charge based on a recentlyacquired battery terminal voltage measurement; a recently acquiredbattery terminal current measurement; and a history of the batterycurrent. The battery state of charge estimate is obtainable during thebattery's course of use.

In another embodiment, a method for estimating the state of charge of abattery includes obtaining a recently acquired battery terminal voltagemeasurement, obtaining a recently acquired battery terminal currentmeasurement, and obtaining a history of the battery current. The methodalso includes estimating a source voltage based on the recently acquiredbattery terminal voltage measurement, the recently acquired batteryterminal current measurement, and the history of the battery current.The method also includes estimating a battery state of charge based onthe source voltage such that the battery state of charge is obtainableduring the battery's course of use.

Various other features, objects, and advantages of the invention will bemade apparent to those skilled in the art from the accompanying drawingsand detailed description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a system in accordance with anembodiment;

FIG. 2 is a schematic illustration of a battery model in accordance withan embodiment;

FIG. 3 is a schematic illustration of a battery model in accordance withan embodiment;

FIG. 4 is an exemplary plot of battery terminal voltage versus state ofcharge;

FIG. 5 is a flowchart illustrating a method in accordance with anembodiment;

FIG. 6 is a graph including a voltage versus time plot and a remainingcapacity versus time plot;

FIG. 7 is a graph including two voltage versus time plots and aremaining capacity versus time plot; and

FIG. 8 is a graph including two voltage versus time plots and aremaining capacity versus time plot.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific embodiments that may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the embodiments, and it is to be understood thatother embodiments may be utilized and that logical, mechanical,electrical and other changes may be made without departing from thescope of the embodiments. The following detailed description is,therefore, not to be taken as limiting the scope of the invention.

Referring to FIG. 1, a system 10 is shown in accordance with oneembodiment. The system 10 includes a battery 12, a controller 14, and anelectronic device 16. The battery 12 may comprise a lead-acid battery.The electronic device 16 may comprise a portable patient monitoringdevice adapted to monitor one or more vital signs such as temperature,pulse rate, blood pressure and respiratory rate. It should, however, beappreciated that the system 10 may alternatively include other types ofbatteries and other electronic devices.

According to one embodiment, the battery 12 is electrically coupled tothe controller 14, and the controller 14 is electrically coupled to theelectronic device 16. Energy from the battery 12 is transferable throughthe controller 14 and to the electronic device 16 in order to power theelectronic device 16. When the battery 12 becomes depleted, it may bere-charged by an external power source 18.

The controller 14 is connected to the positive terminal 22 and thenegative terminal 24 of the battery 12, and is configured to measurebattery terminal voltage V_(t) and/or battery terminal current I_(t) atthe terminals 22, 24 in a known manner. As will be described in detailhereinafter, the controller 14 is also configured to estimate the stateof charge (SOC) of the battery 12 during the battery's course of use.For purposes of this disclosure, SOC refers to a battery's remainingcapacity, and a battery's course of use includes periods wherein thebattery is charging, discharging, and/or idle. The battery SOC estimatecan be transferred from the controller 14 to the electronic device 16for communication to a user.

The controller 14 may estimate battery SOC based on an equation derivedfrom a battery model 19, which is shown in FIG. 2 in accordance with anembodiment. The battery model 19 comprises an electric circuit driven byan internal voltage source Vs, and is adapted to describe theelectrochemical behavior of the battery 12 (shown in FIG. 1). Accordingto one embodiment, the electronic circuit of the battery model 19includes a plurality of resistors R₁-R_(n) disposed in series, and acorresponding plurality of capacitors C₁-C_(n-1) that are each disposedbetween an adjacent pair of resistors. The resistors R₁-R_(n) and thecapacitors C₁-C_(n-1) respectively represent an internal batteryresistance and an internal battery capacitance inherent in manydifferent types of battery. The number of resistors and capacitors canbe increased to improve the precision with which the battery model 20describes the behavior of the battery 12. According to anotherembodiment, the battery model 19 may comprise inductances in place of orin addition to the capacitors C₁-C_(n-1).

Referring to FIG. 3, a simplified battery model 20 is shown inaccordance with an embodiment. The battery model 20 comprises a singlecapacitor C₁ disposed between a pair of resistors R₁ and R₂. Theembodiment of the battery model 20 depicted in FIG. 3 will hereinafterbe described for illustrative purposes, however, it should beappreciated by one skilled in the art that alternate embodimentscomprising additional resistors and capacitors may be implemented in asimilar manner.

It is well known that battery terminal voltage V_(t) can be used toestimate battery SOC if the battery has been idle (i.e., not charged ordischarged) for a period of time. One way of obtaining a battery SOCestimate based on measured battery terminal voltage V_(t) is by using aplot such as that shown in FIG. 4. The plot of FIG. 4 correlates batteryterminal voltage with battery SOC and is available from the manufactureror by compiling test data acquired over a range of battery terminalvoltage values. As an example, if the battery has been idle for asufficiently long period of time, a measured battery terminal voltage of6.0V would indicate an approximately 28% SOC. In other words, it can beestimated that approximately 28% of the battery's capacity remains basedon a battery terminal voltage measurement of 6.0V. One problem with thepreviously described method is that it may be necessary to wait whilethe battery remains idle for a period of several hours before anaccurate estimation of battery SOC can be obtained.

Referring again to FIG. 3, it can be seen that the internal voltagesource V_(s) is generally equivalent to battery terminal voltage V_(t)when the battery has been idle for a period of time. In other words,after the capacitor C₁ reaches voltage level V_(s) and there has notbeen any current passing through the resistors R₁ and R₂ for a period oftime, the voltage at the internal source V_(s) is generally equivalentto the voltage at the terminals V_(t). Accordingly, calculated V_(s)values may be implemented in combination with a plot similar to that ofFIG. 4 to estimate battery SOC in a manner that does not require thebattery to remain idle. If the load current is known or can beestimated, the remaining battery run-time can also be calculated basedon battery SOC. The following will describe a method for calculatingV_(s) such that battery SOC and/or battery run-time can be estimated.

Referring to FIGS. 1 and 3, the controller 14 is configured to calculateV_(s) based on recently acquired battery terminal voltage V_(t) andbattery terminal current I_(t) measurements, as well as a history of thebattery current. For purposes of this disclosure, the history of thebattery current comprises one or more previously acquired batteryterminal current I_(t) measurements. Also for purposes of thisdisclosure, a recently acquired measurement is one obtained generallysimultaneously with its intended use (e.g., within the preceding 5seconds), and a previously acquired measurement is one obtained morethan 30 seconds before its intended use. According to one embodiment,the controller 14 may be configured to calculate V_(s) using theequation: V_(s)=V_(t)−(R₂*I_(t))−(ACC*R₁*(1/B_(tc)) derived from themodel 20. A method implementing this equation to estimate battery SOCand/or battery run-time will now be described.

Referring to FIG. 5, a flow chart illustrating a method 100 forestimating SOC and/or battery run-time is shown in accordance with anembodiment. The individual blocks 102-114 represent steps that may beperformed by the controller 14 (shown in FIG. 1). Those skilled in theart will recognize that the steps 102-114 may be rearranged and/orcombined in ways that preserve the underlying computation.

Referring to FIGS. 3 and 5, at step 102, battery terminal voltage V_(t)is measured at the terminals 22, 24 in a known manner. At step 104,battery terminal current I_(t) is measured at the terminals 22, 24 in aknown manner.

At step 106, the internal voltage source V_(s) is calculated using theequation: V_(s)=V_(t)−(R₂*I_(t))−(ACC*R₁*(1/B_(tc)). An exemplary methodfor calculating V_(s) according to the preceding equation willhereinafter be described in detail. At step 108, battery SOC isestimated. According to one embodiment, battery SOC may be estimatedusing the calculated value of V_(s) obtained at step 106 and the methodpreviously described with respect to FIG. 4. At step 110, batteryrun-time is estimated. According to one embodiment, battery run-time maybe estimated based on SOC obtained at step 108 and a generallyworst-case current draw value. For example, if the most demandingoperation of the electronic device 16 (shown in FIG. 1) draws batterycurrent at a rate of 0.75 amps, this value may be used in combinationwith battery SOC to provide a run-time estimate.

At step 112, the variable ACC is iteratively acquired according to theequation: ACC=((ACC_(previous))*(1−K))+I_(t). For the first iteration,the variable ACC_(previous) may be set to zero. The variable K is abattery constant which can be obtained according to the equationK=1−EXP((−1*(sample rate)/(B_(tc))). The variable B_(tc) is a batterytime constant which can be obtained according to the equationB_(tc)=C₁*R₁. An exemplary method for estimating the battery timeconstant B_(tc) will hereinafter be described in detail. At step 114,the method 100 delays or waits a predetermined amount of time. Accordingto one embodiment, the duration of the delay at step 114 isapproximately 51 seconds. After completing step 114, the method 100returns to step 102.

An exemplary method for calculating or estimating each of the variablesin the equation V_(s)=V_(t)−(R₂*I_(t))−(ACC*R₁*(1/B_(tc)) of step 106will now be described in the order in which they appear. The variableV_(t) may be measured by the controller 14 (shown in FIG. 1) at theterminals 22, 24 in a known manner. The variable R₂ may be obtained fromthe battery manufacturer or may be estimated by measuring the generallyinstantaneous change in battery terminal voltage associated with theapplication or removal of a known load. An exemplary method forestimating R₂ will hereinafter be described with respect to FIG. 6. Thevariable I_(t) may be measured by the controller 14 at the terminals 22,24 in a known manner. The variable ACC is calculated in the mannerdescribed with respect to step 112 of the method 100

The variable R₁ and the battery time constant B_(tc) may be obtainablefrom the battery manufacturer or may be estimated by removing a load andcurve fitting the resultant slope of a voltage vs. time plot. Anexemplary method for calculating the variables R₁, R₂ and the batterytime constant B_(tc) will now be described with respect to FIGS. 6-8.

Referring now to FIG. 6, a load in the form of a 0.29 amp dischargecurrent was applied to a sample battery (not shown) similar to thebattery 12 (shown in FIG. 1). At time T₁ the 0.29 amp discharge currentwas removed from the sample battery and at time T₂ the 0.29 ampdischarge current was re-applied to the sample battery. The curve 40represents the sample battery's terminal voltage versus time in responseto the application of the previously described discharge sequence. Thecurve 42 represents the sample battery's remaining capacity versus timein response to the application of the previously described dischargesequence. It can be seen that the slope of the battery terminal voltagecurve 40 differs from that of the remaining capacity curve 42,particularly between time periods T₁ and T₂, because the batteryterminal voltage curve 40 does not account for the variables R₁, R₂ andthe battery time constant B_(tc) (shown in FIG. 3).

As previously indicated, the variable R₂ (shown in FIG. 3) may beestimated by measuring the generally instantaneous change in batteryterminal voltage associated with the application or removal of a knownload. It can be seen with reference to FIG. 6 that, at time T₁, theterminal voltage (represented by curve 40) increases from approximately5.82 volts to approximately 5.91 volts in response to the removal of the0.29 amp discharge current. Therefore, the variable R₂ can be calculatedusing Ohm's law according to the equation R₂=ΔV/I=(5.91−5.82)/0.29=0.31ohms.

FIG. 7 shows a plot of the previously described curves 40 and 42, and acurve 44. The curve 44 comprises the battery terminal voltage and acorrection factor accounting for the resistance R₂ (shown in FIG. 3).More precisely, the curve 44 is a plot of the quantity(V_(t)−(R₂*I_(t))) versus time in response to the application of thebattery discharge sequence described with respect to FIG. 6.

It can be seen that by incorporating a correction factor adapted toaccount for the resistance R₂, the slope of curve 44 more closelyapproximates that of the remaining capacity curve 42. The slope of thecurve 44 between times T₁ and T₂ is, however, inconsistent with that ofthe remaining capacity curve 42 because the curve 44 does not accountfor the resistance R₁ and the battery time constant B_(tc) (shown inFIG. 3).

An estimate of the variable R₁ can be obtained using the curve 44 byidentifying the change in voltage over time associated with theapplication or removal of a known load. This estimate is predicated onthe assumption that the internal battery capacitance C₁ (shown in FIG.3) will eventually reach voltage level V_(s) such that, over asufficiently long period of time, the change in voltage is attributedexclusively to the resistance R₁. For example it can be seen withreference to FIG. 7 that, during time interval T₁-T₂, the curve 44increases from 5.91 volts to 5.94 volts in response to the removal ofthe 0.29 amp discharge current. This change in voltage over timeassociated with the removal of a known load can be implemented incombination with Ohm's law to calculate R₁ according to the equation:R₁=ΔV/I=(5.94−5.91)/0.29=0.1 ohms.

FIG. 8 shows a plot of the previously described curves 40 and 42, and acurve 46. The curve 46 represents an estimate of the battery's internalvoltage source (V_(s)) based on the battery terminal voltage; a firstcorrection factor accounting for the resistance R₂ (shown in FIG. 3);and a second correction factor accounting for both the resistance R₁ andthe battery time constant B_(tc). More precisely, the V_(s) curve 46 isa plot of the quantity (V_(t)−(R₂*I_(t))−(ACC*R₁*(1/B_(tc))) versus timein response to the application of the battery discharge sequencedescribed with respect to FIG. 6.

It should be appreciated that the equation defining the curve 46includes only one unknown variable (the battery time constant B_(tc)).This unknown variable can be estimated by a trial and error process andby curve fitting the slope of the resultant V_(s) curve. In other words,the estimated B_(tc) variable is that which produces a V_(s) curvehaving a slope most closely matching that of the curve 42. According tothe example illustrated in FIG. 8, the B_(tc) estimate producing theV_(s) curve 46 is 2,000 seconds. As the slope of the curve 46 is highlysimilar to the slope of the curve 42, it can be assumed that theestimated B_(tc) value of 2,000 is accurate.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

1. A battery system comprising: a battery; and a controller operativelyconnected to the battery, said controller being configured to estimate abattery state of charge based on a history of the battery current suchthat the battery state of charge estimate is obtainable during thebattery's course of use.
 2. The battery system of claim 1, wherein thecontroller is further configured to implement a recently acquiredbattery terminal voltage measurement to estimate the battery state ofcharge.
 3. The battery system of claim 1, wherein the controller isfurther configured to implement a recently acquired battery terminalcurrent measurement to estimate the battery state of charge.
 4. Thebattery system of claim 1, wherein the controller is further configuredto estimate the battery state of charge based on an estimated internalresistance of the battery.
 5. The battery system of claim 1, wherein thecontroller is further configured to estimate the battery state of chargebased on an estimated internal capacitance of the battery.
 6. Thebattery system of claim 1, wherein the controller is further configuredto estimate a battery run-time based on the battery state of charge. 7.The battery system of claim 1, wherein the battery comprises a lead-acidbattery.
 8. The battery system of claim 1, wherein the battery comprisesa rechargeable battery adapted for attachment to a remotely locatedpower source.
 9. The battery system of claim 1, wherein the batterycomprises a primary non-rechargeable battery.
 10. A patient monitoringsystem comprising: a battery; a patient monitoring device operativelyconnected to the battery; and a controller operatively connected to thebattery, said controller being configured to estimate a battery state ofcharge based on a recently acquired battery terminal voltagemeasurement; a recently acquired battery terminal current measurement;and a history of the battery current; wherein the battery state ofcharge estimate is obtainable during the battery's course of use. 11.The patient monitoring system of claim 10, wherein the battery comprisesa lead-acid battery.
 12. The patient monitoring system of claim 10,wherein the patient monitoring device comprises a portable patientmonitoring device.
 13. The patient monitoring system of claim 10,wherein the controller is further configured to estimate the batterystate of charge based on an estimated internal resistance of thebattery.
 14. The patient monitoring system of claim 10, wherein thecontroller is further configured to estimate the battery state of chargebased on an estimated internal capacitance of the battery.
 15. Thepatient monitoring system of claim 10, wherein the controller is furtherconfigured to estimate a battery run-time based on the battery state ofcharge.
 16. A method for estimating the state of charge of a batterycomprising: obtaining a recently acquired battery terminal voltagemeasurement; obtaining a recently acquired battery terminal currentmeasurement; obtaining a history of the battery current; estimating asource voltage based on the recently acquired battery terminal voltagemeasurement, the recently acquired battery terminal current measurement,and the history of the battery current; and estimating a battery stateof charge based on the source voltage such that the battery state ofcharge is obtainable during the battery's course of use.
 17. The methodof claim 16, further comprising estimating battery run-time based on thebattery state of charge.
 18. The method of claim 16, wherein saidestimating a source voltage comprises estimating a source voltage basedon an internal resistance of the battery.
 19. The method of claim 16,wherein said estimating a source voltage comprises estimating a sourcevoltage based on an internal capacitance of the battery.